UV/Optical Continuum Variability In AGNs

Abstract

The observed strong UV/Optical excess in many Seyfert 1 galactic nuclei and quasars (QSOs) is well described as thermal radiation from the surface of an optically thick accretion disk surrounding a supermassive black hole (Malkan 1983, Sun and Malkan 1989). This scenario naturally leads to a radially symmetric temperature gradient with the innermost regions the hottest. Thus for a perturbation generated in the inner regions propagating outward, sequential variations from higher to lower frequencies should be expected, aside from the bolometric increase of brightness. However, the extremely intensive International AGN watch (Peterson 1993 and references therein) on NGC 5548 and UV/Optical monitoring campaign on Mkn 335 (Sun et al. 1995) point to opposite conclusions on the time lag between UV and Optical continua, with no lag for NGC 5548 and ∼ 50 days for Mkn 335. We thus began two monitoring programs at Lick Observatory and Mount Stromlo and Siding Spring Observatory (MSSSO) on a sample of Seyfert 1 galaxies with CCD direct imaging to search for more conclusive evidence on the sequential variabilities.